RISC-V: use REG_S/REG_L for mcount

In preparation for rv32i ftrace support, convert mcount routines to use
native sized loads/stores.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Link: https://lore.kernel.org/r/20221115200832.706370-2-jamie@jamieiles.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>